1. Field of the Invention
The present invention relates to a thermally assisted magnetic head comprising a diffraction grating to which laser beams having a plurality of intensity peaks are irradiated.
2. Related Background Art
As recording densities of hard disk devices increase, a further improvement of performance of thin film magnetic heads is demanded. A widely used thin film magnetic head is a composite type thin film magnetic head in which a magnetic detection element, such as a magneto-resistance (MR) effect element, and magnetic recording element, such as an electromagnetic coil element, are layered, and data signals are read from/written to a magnetic recording medium, such as a magnetic disk.
A magnetic recording medium is normally a discrete body in which magnetic particles aggregate, and each magnetic particle has a single magnetic domain structure. Here one recording bit consists of a plurality of magnetic particles. Therefore in order to increase recording density, the magnetic particles must be smaller and bumps on the boundaries of the recording bits must be decreased. If the magnetic particles become smaller, however, a problem is that a drop in heat stability of magnetization occurs due to the decrease of volume.
Heat stability of magnetization is measured by KUV/kBT−. Here KU is magnetic anisotropy energy of the magnetic particle, V is a volume of one magnetic particle, kB is a Boltzmann's constant, and T is an absolute temperature. Decreasing the size of a magnetic particle means decreasing V, and in this state KUV/kBT decreases and heat stability is affected. A possible way to solve this problem is increasing KU at the same time, but increasing KU increases the holding power of the magnetic medium. The write magnetic field strength of a magnetic head, on the other hand, is roughly determined by a saturation magnetic flux density of soft magnetic material constituting the magnetic poles in the head. Therefore if the holding power exceeds an allowable value determined based on the limit of this write magnetic field strength, writing becomes impossible.
As a method to solve the problem of heat stability of magnetization, a thermally assisted magnetic recording method, in which a magnetic material with high KU value is used and at the same time data is written with decreasing the holding power by heating the recording medium immediately before applying the write magnetic field, has been proposed. This method is roughly classified into a magnetic dominant recording method and a light dominant recording method. In the magnetic dominant recording method, the major write element is an electromagnetic coil element, and an emission diameter of the light is greater than the track width (recording width). In the case of the light dominant recording method, the major write element is a light irradiation portion, where the irradiation diameter of the light is roughly the same as the track width (recording width). In other words, the spatial resolution depends on the magnetic field in the case of the magnetic dominant recording method, while the spatial resolution depends on the light in the case of the light dominant recording method.
U.S. Pat. No. 6,944,112 and Japanese Patent Application Laid-Open No. 2005-317178 disclose a hard disk device comprising a light source, such as a semiconductor laser, and a thermally assisted magnetic head. In the case of the hard disk device according to U.S. Pat. No. 6,944,112, a light source is disposed at a position distant from the thermally assisted magnetic head. The light emitted from this light source is irradiated onto an optical wave guide in the thermally assisted magnetic head, and is guided to a near field light generation unit (plasmon antenna) disposed on a medium facing surface. The magnetic recording medium is heated by the near field light generated from the near field light generation unit, thereby thermally assisted magnetic recording is performed.
In the case of the hard disk device according to Japanese Patent Application Laid-Open No. 2005-317178, a semiconductor laser, which is a light source, and a thermally assisted magnetic head, are integrated. Thermally assisted magnetic recording is performed by directly irradiating the emission light from the light source onto the magnetic recording medium for heating.
The laser beams contact the diffraction grating, deflect there, propagate through the wave guide, and are partially condensed to the plasmon antenna. However in the case of conventional head, a magnetic pole for recording penetrates through the center of the optical wave guide, and interrupts laser propagation by absorbing or scattering the laser beams. This magnetic pole for recording absorbs or scatters the laser beams, which makes it difficult for laser beams with sufficient intensity to be irradiated onto the plasma antenna, and makes the intensity of the near field light generated from the plasmon antenna insufficient.